I suggest you build the trap underground. This is easier tham having to build a really tall tower(and risk falling yourself. I've fallen so many times trying to build a good mob grinder) to have the monsters fall. Dig a really deep hole and at the top of it build a large room that is 2 blocks high. Make sure its large and has no windors or torches. The only tricky part about this is being able to co-ordinate the place where the monsters fall and the opening where you are going to collect the goodies. This trap will kill 100% of all the mobs as long as you dig the hole deep enough(you can even add some cactus to the bottom of your shaft/hole to make sure that the mobs are dead) This trap also works on minecraft pocket edition.

I suggest you build the trap underground. This is easier than having to build a really tall tower  (and risk falling yourself). Dig a really deep hole and at the top of it build a large room that is 2 blocks high. Make sure it's large and has no windows or torches. The only tricky part about this is being able to co-ordinate the place where the monsters fall and the opening where you are going to collect the goodies.

This trap will kill 100% of all the mobs as long as you dig the hole deep enough. You can even add some cactus to the bottom of your shaft/hole to make sure that the mobs are dead. This trap also works on minecraft pocket edition.